In a spindle motor for rotating a recording medium in an information recording/reproducing device such as a hard disk drive, various kinds of thrust dynamic pressure bearings are proposed that generate dynamic pressure by means of spiral grooves or herringbone grooves with intermediate bends, as a means for suppressing axial runout as well as for supporting an axiswise load, of the rotor. (Refer to Japanese Patent Unexamined Publication No. 2001-173645 (Paragraph 6, FIG. 3), No. 2003-113837 (Paragraph 7, FIG. 1), and No. H10-73127 (Paragraph 12, FIG. 8), for example.)
FIG. 12A illustrates the makeup in principle of a conventional thrust dynamic pressure bearing. The bearing is composed of bearing surface 11 of rotating-side member (rotor) 10, and bearing surface 21 of fixed-side member 20, where both surfaces mutually face in the direction of rotation center axis 1, having a minute interspace filled with lubricating oil 50 therebetween. The outer circumference of the minute interspace is provided thereon with a sealing section forming air-liquid boundary surface 51 for lubricating oil 50 and air. At least one of bearing surface 11 of rotating-side bearing member 10 and bearing surface 21 of fixed-side bearing member 20 is provided thereon with dynamic pressure generating grooves 30. Further, land 40 is formed adjacent to dynamic pressure generating groove 30.
FIG. 12B illustrates another makeup in principle of a conventional thrust dynamic pressure bearing. In the same way as shown in FIG. 12A, the bearing is composed of bearing surface 11 of rotating-side member (rotor) 10 , and bearing surface 21 of fixed-side member 20, where both surfaces mutually face in the direction of rotation center axis 1, having a minute interspace filled with lubricating oil 50 therebetween. The outer circumference of the minute interspace is provided thereon with a sealing section forming air-liquid boundary surface 51 for lubricating oil 50 and air.
As the shape of dynamic pressure generating groove 30, spiral groove 35 shown in FIG. 13 and herringbone groove 31 shown in FIG. 14 are known. Dynamic pressure generating grooves 31, 35, which are recesses; and lands 41, 45, which are projections with the roughly same shape as the dynamic pressure generating grooves, are formed alternately at a given pitch. Groove width G and land width L hold G=L (in patent literature 1), or G<L (in patent literatures 2, 3), where groove width G and land width L are length or angle of an arc formed with circle 2 with an arbitrary radius assumed to be drawn on the bearing surface, and dynamic pressure generating grooves 31, 35 and lands 41, 45, all intersecting.
The cross sections of the dynamic pressure generating grooves along circle 2 with an arbitrary radius are shown in FIGS. 15A, 15B. When using a method such as etching, coining, electrolytic processing, or electric discharging, to form dynamic pressure generating grooves, lands 41, 45 generally have a trapezoidal shape, and corners C of lands 41, 45 and the grooves are often made rounded, as shown in FIG. 15A. Land width L in such a case means a region where the cross-sectional profile of a land is higher than the elevation control line, which is half of groove depth H; and width G, lower than this elevation control line. Meanwhile, when forming dynamic pressure generating grooves with an NC lathe and further smoothing the top of a land formed in FIG. 15A with a flat-surface grinding machine or the like, the corner at the top of the land could be sharp as shown in FIG. 15B. Land width L in such a case is to be the width of the flat part at the top of the land.
Typical thrust dynamic pressure generating grooves formed by electrolytic processing or electric discharging are illustrated in drawings of patent literature 3 and others. Their representative example is shown in FIG. 16A. The upper and lower part of fixed-side bearing member 320 fixed to fixed axis 300 (shown in FIG. 16B later) are provided thereon with herringbone grooves 331, which are recesses. Herringbone groove 331, if formed by electrolytic processing or the like, has roughly the same circumferential length from the inner circumference through the outer circumference. Land 341 adjacent to herringbone groove 331 is formed as an unprocessed part by electrolytic processing. The most inner and outer circumferences of fixed-side bearing member 320 compose ring-shaped recesses 350, 351 by electrolytic processing, cutting, or the like, in the same way as in herringbone groove 331, so that lubricating oil smoothly moves inside the thrust bearing. The sectional view of the substantial part of the bearing using this fixed-side bearing member 320 for a thrust bearing is shown in FIGS. 16B, 16C. FIGS. 16B, 16C show states of the motor stopped and steadily rotating, respectively. In a state of the motor stopped, the top surface of fixed-side bearing member 320 contacts rotating-side thrust plate 310. Meanwhile, in a state of the motor steadily rotating, a dynamic pressure occurs between member 320 and plate 310, causing them to be floated and spaced by a given height. Here, when shifting from a stopped state to a rotating state, or vise versa, lubricating oil moves back and forth vertically across the thrust bearing through herringbone groove 331 and ring-shaped recesses 350, 351, as shown by direction 353 (outline arrow ).
Meanwhile, with the popularization of personal digital assistances of recent years, downsizing, slimming down, and low power consumption are demanded in information recording/reproducing devices such as a hard disk drive to be mounted on a personal digital assistance. Consequently, slimming down, implementing of highly accurate rotation free of axis runout, and low power consumption are essential to a spindle motor for rotating a recording medium of an information recording/reproducing device.
In a spindle motor used for a conventional, relatively large hard disk drive such as 3.5-inch or 2.5-inch types, its rotation axis can be made long. Therefore, as a result that two-tiered, radial dynamic pressure bearings are arranged around the rotation axis, and the axis is supported at two points, upper and lower, rigidity (referred to as “inclination-resistant rigidity” hereinafter) has been secured against disturbance moment torque, which tends to generate axis runout by fluctuating pivotal inclination. Meanwhile, in a spindle motor for a small, slim hard disk drive of 1.8-inch type or smaller, the rotation axis must be made short for slimming down. Therefore, it is difficult to arrange two-tiered, radial dynamic pressure bearings around the rotation axis. Consequently, it is difficult to secure inclination-resistant rigidity for suppressing pivotal runout by a radial dynamic pressure bearing.
Even if two-tiered arrangement is achieved, inclination-resistant rigidity in radial bearings is proportional to the approximate square of the pitch between the two radial bearings. This causes inclination-resistant rigidity to become significantly low in a slim motor applied particularly for a hard disk drive with a thickness of 5 mm or less.
Under the circumstances, increasing inclination-resistant rigidity of the thrust dynamic pressure bearing is required to suppress pivotal runout by a thrust dynamic pressure bearing instead of a radial one. Meanwhile, bearing loss torque of the dynamic pressure bearing needs to be decreased to reduce power consumption. In other words, a thrust dynamic pressure bearing for a slim spindle motor with low power consumption and high rotation accuracy requests higher inclination-resistant rigidity and lower bearing loss torque than a conventional thrust dynamic pressure bearing.
Methods for processing thrust dynamic pressure grooves include etching, coining, and electrolytic processing. When forming thrust dynamic pressure grooves with such a method and incorporating them into a motor, eccentricity from the rotation center and variation in groove width assuredly result. Here, if the design goal is set so that the ratio of groove width G to land width L will be 1:1, the ratio randomly fluctuates even on the same radius due to eccentricity and/or variation in processing, resulting in G>L or G<L depending on the location of a dynamic pressure groove. This leads to the degree of concentration (pump-in/pump-out characteristic) changing according to a rotation phase when generating pressure by concentrating lubricating oil on a given position owing to land effect caused by the dynamic pressure grooves. In a state of G:L=50:50, the rotation axis tends to incline due to the weakest inclination-resistant rigidity as described later, thus inducing fluctuation in fluid level and/or vibration of the surface, of lubricating oil near the thrust bearing. Lubricating oil will increases and/or decreases between the facing surfaces of the thrust bearing, in a transitional condition such as one where the floating level of the thrust bearing changes from zero to a given value, particularly when starting the motor; or when the rotation speed rapidly fluctuates during operation in the motor for an optical disc drive. At this moment, when ratio G:L is approximately 50:50 and additionally randomly disperses depending on location or rotation phase of a thrust dynamic pressure groove, the rotation axis tends to incline due to its low inclination-resistant rigidity, preventing lubricating oil from being normally supplied and discharged depending on phase. If the fluid surface vibrates and/or supplying and discharging of lubricating oil malfunction, lubricating oil will not fill the space between the thrust bearings, and consequently air will be involved in the bearing, and/or lubricating oil will leak from the bearing. An experiment proves that such phenomena prominently occur if the randomly fluctuating component ΔG of groove width G exceeds approximately 3% of (G+L). In this way, the bearing involving air therein causes problems, namely generating rotation fluctuation component, decreasing the bearing rigidity, and additionally shortening the bearing life if lubricating oil leaks from the bearing.